AQA Biology Unit 3


B3.1- Osmosis

Osmosis is the diffusion of water. Water molecules are very small, so can move through a partially permeable membrane when other, larger molecules (for example, sugar) cannot. Like diffusion, water molecules will move from an area of high concentration to an area of low concentration. 

Osmosis and Plant Cells

  • Surroundings less concentrated- water in cell increases, pressure increases, rigid cell (turgid)
  • Surroundings equally concentrated- no net movement of water, no change
  • Surroundings more concentrated- water in cell decreases, cell wall collapses (plasmolysed)

Osmosis and Animal Cells

  • Surroundings less concentrated- water in cell increases, cell swells, may burst (lysis)
  • Surroundings equally concentrated- no net movement of water, no change
  • Surroundings more concentrated- water in cell deceases, cell crinkles (crenated)
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B3.2- Sports Drinks

Water: protects organs: regulates temperature; keeps skin, mouth, nose and lungs moist; carries mineral ions; lubricates joints; carries dissolved substances (eg oxygen, carbon dioxide)

  • Dissolved substances: lower than in body
  • What it does: moves water from gut into blood quickly
  • Purpose: hydration
  • Dissolved substances: same level as in body
  • What it does: some water moves into blood, some sugar is absorbed
  • Purpose: hydration and fuel replacement
  • Dissolved substances: higher than in body
  • What it does: provides high levels of sugar absorption
  • Purpose: supplying fuel (causing a sugar rush)
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B3.3- Active Transport

Active transport- molecules moving from a low concentration to a high concentration (against the concentration gradient). Requires energy from respiration (obtain from molecule ATP) and needs a protein carrier in the membrane

Examples of active transport:

1. Sodium ions out of nerve cells in the human body

2. Absorption of mineral ions by root cells (root hair cells have long extensions with protein carriers in all of their membranes to help with active transport in soil)

3. Absorption of sugars by cells (glucose is absorped by cells lining the gut through active transport, and sodium is often absorpbed at the same time hence why sports drinks contain sodium ions)

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B3.4- Exchange Surfaces

The surface area : volume falls as a cell or organism increases in size. This is important because they need exchange of important molecules such as oxygen to be able to efficiently move into and out of the cell/organism. When the ratio is low, exhchange becomes inefficient. Some exchange systems are adapted to overcome this issue

Features of exchange systems

  • Large surface area: provides larger area for diffusion
  • Thin surface: short distance for diffusion to take place over
  • Blood supply: maintains high concentration gradient
  • Turnover: maintains high concentration gradient

The small intestine is where the nutrients from digested foods are absorpbed. To maximise the exchange of the wall of the small intestine, it has several adaptations. Firstly, it is lined with millions of villi to increase its surface area. These villi each contain a lacteal at the centre to absorb fats and maintain a high concentration, a dense capillary network for the same reason, and an epithelial membrane only one cell thick to provide a short diffusion pathway. 

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B3.5- Gaseous Exchange in the Lungs

Air breathed in > warmed and filtered in mouth and nose > passes down trachea > trachea branches (called bronchus) into lungs > bronchus branches into smaller bronchioles > bronchioles have alveoli on their ends

Adaptations of the alveoli

  • Surface area- the presence of alveoli greatly increases the surface area of the lungs
  • Thin wall- short diffusion pathway
  • Dense blood supply- maintains concentration gradient
  • Moist lining- allows gases to diffuse 

Ventilation is the act of inhaling and exhaling

Inhaling- 1) Intercostal muscles contract, ribcage moves up and out, thorax expands. 2) Diaphragm contracts, also expanding thorax. 3) Volume in thorax has increased, pressure has decreased. 4) Air rushes into lungs to increase pressure

Exhaling- 1) Intercostal muscles relax, ribcage falls. 2) Diaphragm relaxes, reducing thorax size. 3) Pressure in lungs has increase. 4) Higher pressure forces the air out

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B3.6- Exchange in Plants

Two major exchange surfaces in plants: roots and leaves

Why plants need water

  • photosynthesis
  • cooling effect (when evaporated)
  • makes cells firm, supports plant
  • transports dissolved minerals

Transpiration- water flowing from the roots up the stem and into the leaves, where it evaporates

Controlling water loss: stomata

Stomata are on the lower leaf surface because this gets less sun, so less water can eavporate. Each stoma can be opened or closed. They are open when the plant is photosynthesising, and closed during night. The stomata do not open when the plant has little water. The stomata are controlled by guard cells which swell when there is a lot of water in the plant. This causes the gap to open and water to leave. 

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B3.7- Rates of Transpiration

Factors increasing rates of transpiration

  • Increasing light intensity

Stomata close in the dark and open in the light. If the light intensity is increased, more stomata will be open, allowing more water out of the plant

  • Increase in temperature

The higher the temperature is, the faster the particles of air and water will move. This increases the likelihood of any one particle leaving the plant through the stomata

  • Increase in air movement

The faster the air movement is, the faster water molecules will be moved from the surface of the leaf. This maintains a high concentration gradient, so transpiration rate is high

  • Decrease in humidity

Higher humidity creates a low concentration gradient, which decreases the rate of transpiration

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B3.8- The Circulatory System

Blood is the main transport system in the human body. It carries oxygen and nutrients to where they are needed. Blood is a tissue made up of red and white blood cells, plasma and platelets. 

Artificial blood

Volume increasers- saline may be used to maintian normal blood pressure when someone has lost a lot of blood, because maintaining normal blood pressure means the remaining haemoglobin in the real blood will be able to provide the tissues with enough oxygen to sustain a motionless patient

Oxygen carriers- if the blood loss was more than 2/3, there would not be enough remaining blood carrying oxygen, so the person would require an oxygen-carrying substitute. Some of these substitutes carry and release oxygen, whilst others contain encapsulated haemoglobin. 

Uses of artificial blood products: treating war casualties, treating patients regardless of blood type, giving transfusions when the blood cannot be screened, treat trauma patients, store for a long time, immediately restore oxygen-carrying capacity (real blood takes 24 hours)

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B3.9- The Heart

The heart is a double pump (because humans have a double circulatory system) with four chambers. 

Deoxygenated blood (from the body) is pumped from the vena cava into the right atrium. The walls of the atrium contract, forcing the blood through a valve and into the right ventricle. This blood then leavesthe heart via the pulmonary artery and travels to the lungs to be oxygenated. It is then transported through the pulmonary vein into the left atrium of the heart, which contracts to force the blood into the left ventricle. The blood exits the heart through the aorta, and the blood carries oxygen to the organs in tissues in the body. 

Artificial hearts

Valves stop blood from flowing backwards in the hear. Because they have no capillary blood supply, valves can be easily transplanted without risk of rejection. Valves can be replaced with those of other animals, or synthetic heart valves. 

Artificial hearts are very smooth and seamless so that they do not cause blood clotting. They are often used when waiting for a heart transplant. A disadvantage of artificial hearts os that the wires protrude through the skin. 

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B3.10- Arteries and Veins


  • high pressures
  • eslatic fibres to stretch and recoil (smooth flow)
  • muscle fibres to withstand high pressure
  • narrow lumen 
  • pulse


  • low pressures
  • wide lumen
  • fwer muscle and eslatic fibres
  • surrounded skeletal muscle to push blood through
  • valves to prevent backflow

Stents are used if the arteries narrow due to a build up of saturated fat. This reduces the bloodflow, which can cause clotting. A stent is a small mesh tube that expands inside an artery to ease the flow of blood. 

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B3.11- Capillaries

When arteries reach body tissues, they brain into narrow vessels called arterioles, which branch into even narrower vessels called cappilaries. Capillaries allow exchange of substances between blood and tissues. 


  • Wall only one cell thick
  • Low pressure so as not to damage walls
  • Narrow lumen- one blood cell at a time
  • Form networks to increase concentration gradient and surface area


  • oxygen (aerobic respiration)
  • glucose (respiration)
  • amino acids (for making proteins)
  • urea (breakdown of amino acid/haemoglobin in liver)
  • hormones 
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B3.12- Blood

Blood is for transport, regulation and protection. 


Red blood cells carry oxygen from lungs to heart, then heart to tissues. The plasma carries soluble products of digestion, urea from liver to kidneys, carbon dioxide from organs to lungs and hormones from glands to target cells. 


  • Body temperature- distributes heat from respiring muscles and liver cells to skin/other organs
  • pH in tissues- plasma proteins resist changes in pH


Blood clots at wounds to prevent pathogens from entering. When pathogens do enter, white blood cells have defences against them.

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B3.13- Blood cells

Red blood cells

They are created in bone marrow and lose their nucleus before entering the bloodstream, giving them a concave shape. They contain haemoglobin, a protein containing iron which bonds with oxygen to form oxyhaemoglobin. A red blood cell lives for arounfd four months before being broken down in the liver.

White blood cells

Unlike red blood cells, these do have a nucleus. There are several different types of white blood cell, and all form part of the body's defence system against microorganisms. Some ingest them, some produce antibodies and some produce antitoxins. 


These are small cell fragments. They have no nucleus. A platelet lasts for about a week, and whilst it is in the blood, it helps to clot at the site of wounds. Platelets, like red blood cells, are constantly being made in the bone marrow. Old platelets are destroyed by phagocytes in the liver. 

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B3.14- Transport in Plants

There are two major functions of the vascular bundles in plants: transport and support. 

Xylem: dead cells, stacked to form hollow tubes, transports water and dissolved minerals from roots to shoots

Phloem: living cells, stacked into tubes, transport food substances made in the leaf to all other parts of the plant

Transpiration Stream

Root hair takes in water and dissolved minerals > they move through cells to the xylem > water and minerals move up the xylem through stem into leaves > water moves into leaves and evaporates through stomata as water vapour


Ploem transports sugars made by photosynthesis in the leaf (source) to areas of the plant for storage or growth (sink). 

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B3 Part 1 Catch up

  • Dissolved substances move in/out cells by diffusion or active transport. Water moves by osmosis
  • Water keeps cells hydrated. Exercise uses glucose and causes loss of water and ions in sweat which can be replaced by sports drinks
  • Active transport moves substances across cell membranes against the concentration gradient
  • As organisms get bigger their surface area to volume ratio becomes smaller so exchange surfaces are needed
  • Exchange surfaces are thin, have a large surface area and have a concentration gradient
  • Breathing brings oxygen into the lungs, which is exchanged at the alveoli
  • Plants lose water by transpiration
  • Factors affecting transpiration are air movement, light, temperature and humidity
  • Large complex organisms need a circulatory system to transport substances
  • Humans have a double circulatory system. Artificial valves/hearts treat some issues
  • Arteries carry blood away from the heart, veins into the heart
  • At body tissues, substances are exchanged between blood, capillaries and cells
  • Blood is for transportation, regulation and protection
  • Red blood cells are for oxygen transportation, white for defence; platelets are for clotting
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B3.15- Keeping Internal Conditions Constant

Internal conditions that are controlled:

  • pH
  • water/ion content
  • temperature
  • blood sugar levels
  • Carbon dioxide-  It has to be taken out of the body because it would lower the pH of blood, tissue fluid and cells otherwise. It would interfere with enzyme action 
  • Urea- The liver breaks down excess amino acids and makes ammonia. This is very alkaline, so reacts with carbon dioxide to make urea. This is less toxic than ammonia, so can be taken to the kidneys to be removed
  • Water and ions- If there was too much, cells would take in too much water by osmosis and burst, and the opposite if too little. It is released through urine, sweat and exhalation
  • Temperature- This must be regulated because enzymes would not collide with substrates if too low, and would denature if too high
  • Blood sugar- Cells need a continuous supply of glucose for respiration. If there was too much, water would leave the cells by osmosis. Too little would cause fatigue
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B3.16- The Kidney

A healthy kidney produces urine by: 

  • filtering the blood
  • reabsorbing the sugar
  • reabsorbing as much water/ions as needed
  • releasing urea, excess water and ions are urine

Blood flows into the kidney through the renal artery. The above process takes place, then the filtered blood is sent back to the heart via the renal vein. The waste urine passes down the ureter to the bladder

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B3.17- Renal Dialysis

People who suffer from kidney failure have two treatment options: transplant or a dialysis machine. 


The damaged kidney is replaced by a healthy one from a donor. However, the immune system will recognise the antigens on its surface as foreign, so immunosuppressant drugs have to be taken to prevent rejection. 


The person's blood flows through a machine containing a partially permeable membrane. On one side of the membrane is dialysis fluid, which contains the same amount of useful substances as in the blood, but no urea, so this diffuses across the membrane. This occurs several times over a period of around six hours, three to four times a week. It is usually done whilst the patient is asleep. 

People with renal failure must carefully monitor their fluid, salt and protein intake. 

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B3.18- Kidney Transplants


  • Can survive with only one kidney- better for donor and recipient
  • Does not require repeated hospital visits
  • Can live an almost normal life
  • Cheaper in the long term 


  • Suitable donors must be found- matching tissue types
  • Immunosuppresent drugs increase risk of common illnesses
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B3.19- Regulating Body Temperature (Overheating)

Thermoregulation means keeping the body within safe limits. It is carried out by the thermoregulatory centre in the brain. It does it by monitoring the blood that flows through it every two or three minutes. If the blood passing through the thermoregulatory centre is too high or low, the brain stimulates the body to make adjustments. 

Actions taken when overheating

  • More blood flows to skin so heat can dissipate out the body
  • Nerve impulses stimulate sweat glands to release sweat, which evaporates with heat
  • Vasodilation occurs

Vasodilation is when nerves stimulate blood vessels supplying the capillaries, causing them to dilate. More blood can then flow through the capillaries, and escess heat from the blood is lost through the skin to surroundings. 

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B3.20- Regulating Body Temperature

If the thermoregulatory centre detects too low a temperature in the blood, it prompts the body to take other actions to warm up. 

Actions taken to warm up

  • Reduced sweating so heat is not lost by evaporation
  • The muscles contract rapidly (shivering) which causes the muscle cells to respire, releasing some heat energy as they do so
  • Vasoconstriction occurs 

Vasoconstriction is when the aterioles that supply the capillaries get narrower. Less blood flows into the capillaries, so less heat can be lost by radiation. This protects the core of the body, where the most important internal organs are. This is why people often lost fingers or toes (extremities) when exposed to very cold conditions for a prolonged period of time- the vital organs are taking the heat energy that those cells need to live. 

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B3.30- Regulating Blood Glucose Levels

The glucose levels must be regulated because glucose is the energy source for every cell in the body. Much like for temperature, the glucose levels in the blood are measured in the pancreas by blood flowing through it. 

Glucose level too high

The pancreas secretes the hormone insulin. This is carried to all cells, and causes them to take up more glucose from the bloodstream, either to store it or use it for respiration. When it is stored, it is stored as a molecule called glycogen, which is a large carbohydrate. 

Glucose level too low

The pancreas can secrete another hormone called glucagon if the blood glucose levels are too low. This causes the cells that store glucose as glycogen (see above) to break it back down into glucose and release it into the blood. 

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B3.22- Type 1 Diabetes

Type 1 diabetes is a disease in which the pancreas does not produce enough insulin, causing the blood glucose levels to be too high. This is because the immune system has unintentionally destroyed the cells that create insulin. 

There are several ways of treating type 1 diabetes:

  • controlling diet (regular meals with fibre and complex carbohydrates, little sugar)
  • regular exercise
  • regular monitoring of blood glucose levels and injecting insulin 
  • avoiding alcohol
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B3.23- Human Populations

There has been a massive increase in the world population since 1900 due to:

  • improved diet
  • improved hygeine
  • improved healthcare
  • lowered infant mortality

This has had a number of effects, including: 

  • food shortages
  • more land used for farming/building
  • more pollution
  • non-renewable resources being used up faster 

Humans can manage their resources by:

  • replacing them where possible (eg planting new trees)
  • avoiding overuse (eg placing quotas on fishing)
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B3.24- Pollution

Air pollution

  • Sulfur dioxide and nitrogen oxide- caused by burning fossil fuels, causes acid rain
  • Carbon dioxide- caused by burning fossil fuels, causes acid rain and global warming
  • Methane- caused by cows and rice fields, causes global warming
  • Smoke- caused by burning fuels, causes bronchitis and reduced photosynthesis
  • CFCs- caused by aerosols and refrigerator coolants, causes ozone layer holes

Water pollution 

  • Untreated sewage- caused by sewage works, causes disease and eutrophication
  • Fertilisers- causeed by farming, causes eutrophication
  • Pesticides and herbicides- caused by farming, causes toxicity which damages food chains
  • Toxic chemicals- caused by factories and mining, causes poisoning or toxicity in food chains
  • Detergents- caused by domestic use, causes death of valuable microbes
  • Oil- caused by tankers and piplines, causes bird and fish deaths
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B3.25- Deforestation

Large scale deforestation in tropical areas has increased the amount of carbon dioxide being released into the atmosphere, as well as reducing the amount being taken in by trees and stored. 


  • 1) Slash and burn- when forests are cleared for farms they are cut and burnt, releasing CO2
  • 2) Effect on global gases- carbon dioxide is increased by burning, increased during decomposition, and increased as less photosynthesis occurs
  • 3) Land for rice and cattle- deforestated areas are used for cows, which produce methane
  • 4) Land for biofuel crops- deforested land is used for carbon neutral biofuels
  • 5) Loss of biodiversity- the diverse forest is replaced with one crop, which is only one habitat
  • 6) Loss of future resourcess- many species which may be very useful to humans go extinct
  • 7) Soil erosion- the removal of trees leads to this because there are no roots to hold the soil 

Peat is dead moss that is in anaerobic conditions and so has not been allowed to decay. It has a lot of carbon dioxide stored in it, so when peat is removed from the bogs and used as compost, the decay process starts up again because it has access to air. Many gardeners now use peat-free compost. 

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B3.26- Global Warming

Global warming- an overall increase in global temperatures

Causes of global warming

The two main gases that cause the greenhouse effect are carbon dioxide and methane. They reflect heat back towards earth instead of letting it radiate into space. 

Effects of global warming

  • Melting ice caps
  • Climate change (stress to environment)
  • Changing migration patterns
  • Changing distribution patterns
  • Reduced biodiversity

Large bodies of water absorb a lot of carbon dioxide, so oceans, lakes and ponds reduce the carbon dioxide in the atmosphere either by dissolving it directly or the phytoplankton that live in water photosynthesising 

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B3.27- Biofuels

Advantages of biofuels

  • Reduces fossil fuel consumption
  • Carbon neutral- no additional carbon dioxide
  • No smoke (particulates) produced

Disadvantages of biofuels

  • Habitat loss to grow the plants- leads to extinction

Biogas is made by the fermentation of carbohydrates in plant material and sewage. It is a useful fuel as it burns in oxygen. It produces fewer particulates than petrol or diesel when burned, but also less energy. Additionally, many different bacteria are needed to produce it, and in anaerobic conditions. 

Bioethanol is alcohol produced from plant material and yeast. When mixed with petrol (to become gasohol) it can be used as a fuel. This is useful in coountries such as Brazil, which has no oil reserves for fuel, but plenty of waste sugar canes. 

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B3.28- Microbes and Food Production

Mycoprotein is a vegetarian, high-protein food product that is produced from a fungus that can convert waste plant materials into food. 

Making mycoprotein

  • The fungi are placed in a tank filled with water and glucose (food for the fungus)
  • The pH and temperature are monitored to provide the best growth rate for the fungus
  • Paddles stir the mixture so the fungus does not settle at the bottom
  • Oxygen is added to allow the fungus to respire
  • The mycoprotein produced is extracted, then heated to a 65 degrees to remove excess nucleic acid. It is centrifuged to reomve water, egg is added to bind it and then the mycoprotein is frozen to resemble meat. 
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B3.29- Food Chains and Production

Maximising energy transfer in food chains

In battery farming, the animals are kept in small cages to avoid movement and energy loss. They are also kept indoors in a warm environment, so no energy is wasted trying to heat themselves. This is often called energy efficient farming because it saves energy that is then passed on to the consumer. 


  • Less energy transferred out of chain, so more for human consumption
  • Less labour intensive
  • Less risk of attack from predators
  • Cheaper production costs


  • Greater risk of disease
  • Inhumane and produces poorer quality product
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B3.30- Fishing

Modern fishing techniques have made fishing incredibly effective. Technology such as sonar to locate fish, sophisticated net designs to prevent escaping and well designed boats have led to a huge increase in the amount of fish being caught, which has put the status of some fish populations into critically low numbers. 

Protecting fish stocks

  • Net size- the size of net holes has been increased to allow smaller fish through. They can then continue to survive and breed, so the population can recover
  • Quotas- governments have set limits on the number of fish of each species that are allowed to be caught

Fishing is being reduced to a sustainable level, but these changes have meant that fishing communities have suffered economically, and there are higher levels of unemployment in the industry

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B3 Part 2 Catch up

  • Living organisms produce waste products (eg, carbon dioxide, urea, amino acids) that must be disposed of
  • Body temperature, glucose levels, water and ion content must be kept within narrow ranges
  • Kidneys filter urea and excess water/ions from the blood
  • Renal failure can be treated with dialysis or a transplant
  • In a transplant the recipient and donor must be matched for tissue types to avoid rejection
  • Sweating and vasodilation prevent overheating
  • Shiering and vasoconstriction prevent overcooling
  • Insulin lowers blood sugar levels, glucagon increases blood sugar levels
  • Type 1 diabetes is defined by the pancreas not making enough insulin to lower blood sugar 
  • The human population is dramatically increasing, meaing more resources being used and more pollution being produced
  • Agriculture and industry pollute the air and water in numerous ways
  • Deforestation increases the carbon dioxide in the atmosphere, as well as reducing the amount that is taken up by trees in photosynthesis
  • Biofuels could replace fossil fuels and reduce the greenhouse effect but they use up land
  • Knowledge of how energy passes along food chains helps energy efficient farming methods
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